Optimization of the InGaP Top ${\it pn}$ Junction for a New Triple Tandem Solar Cell Design

The solar cells with highest efficiencies are the tandem cells with three or four pn junctions in series, connected by tunnel diodes. The current produced by the stacked pn junctions is limited by the smallest one. Therefore, to optimize the full solar cell efficiency it is crucial to match the current produced by the different junctions. This issue has been extensively investigated over the years. In the most usual case of three pn junctions, the current is limited by the middle junction. Recently, the use of multiple quantum wells to further improve the current generated at the intermediate pn junction was proposed. When such a middle junction is used, new optimization of the top cell is required to reach a better current matching. In this work we have used a commercial software, Comsol, to optimize the top solar cell for a triple junction structure to be used in space applications, meaning subjected to the AM0 spectrum. The newly designed solar cell is based on an InGaP pn junction with AlGaInP n-doped window. The layers are lattice matched to GaAs and to Ge, which is the material used for the bottom pn junction. These materials were chosen based on the fact that they are more resistant to radiation, which is of paramount importance for use in satellites. The first step to fabricate the designed solar cell is the optimization of each individual layer. The different InGaP and AlGaInP semiconductor layers of the designed solar cell have been grown by metalorganic vapor phase epitaxy at 675 oC. The alloys' composition was calibrated. High doping levels of InGaP were achieved. However, difficulties in reaching a doping level of the AlGaInP window layer of around 1x1018cm−3, as required, were faced and should be discussed. Additionally, luminescence and x-ray diffraction data of the grown samples will be presented.